Energy, waste and resources – three sides of the same coin?

A report of the Environmental Chemistry Group 2012 Distinguished Guest Lecture and Symposium, held in the Chemistry Centre at Burlington House on Wednesday, March 14th 2012.

The 2012 Distinguished Guest Lecture was given by Professor Paul Williams (University of Leeds), and around sixty people attended. Biographical information for the DGL and supporting speakers together with their slides and abstracts for the presentations are available at http://www.rsc.org/Membership/Networking/InterestGroups/Environmental/2012-distinguished-guest-lecture.asp.

The first paper (“Energy from Waste: a policy perspective”) was delivered by James Cooper, Head of Energy-from-Waste Policy, DEFRA. He described the UK government’s role and ambitions with respect to energy-for-waste as being part of the strategy “to achieve a more sustainable approach to the use of materials and an improvement in the services offered to households and businesses in relation to waste collection and disposal whilst delivering environmental benefits and supporting economic growth.” Household recycling rates have increased from 11% in 2000/2001 to 40% currently. Energy from Waste (EfW) is seen as one way to hit increasingly stringent targets for reduced waste disposal via landfill. More EfW will also help to meet local and national renewable energy targets. To do this consistently it is expected that efficiencies will have to increase to balance a predicted decrease in feedstock: “More energy out of less waste” is needed. The waste market in the UK accounts for ~0.7% of GDP and in a green economy there will be opportunities to grow this sector; the production of reformed biofuels from waste incineration is one such possibility receiving close attention. To be successful, all next-generation EfW processes must be able to respond and adapt to continuing changes in environmental legislation regarding emissions, products and feedstock mix. For further information see the Government Review of Waste Policy in England 2011; http://www.defra.gov.uk/publications/2011/06/14/pb13540-waste-review/.

The second speaker, David Brignall (Wardell Armstrong), spoke on “Pyrolysis and gasification – planning and environmental impacts”. He described how the use of gasification [a process that converts carbon-based materials at temperatures above 700 °C to produce syngas (a gas mixture containing varying amounts of carbon monoxide and hydrogen) and solid residues] and pyrolysis (the thermochemical decomposition of organic materials at elevated temperatures without oxygen, producing chars, oil and syngas) satisfied the need for reduced landfill disposal, renewable Combined Heat and Power (CHP), and local waste strategy solutions. Complications arise because any demonstration of commercial viability is site-based and complex; pyrolysis and gasification processes are therefore perceived as a high long-term investment risk. These problems are exacerbated by the difficulties of identifying long-term feedstock supplies and ensuring that any plant that is built can easily be adapted to respond to changes in the legislative framework. Public resistance to the building of incineration plants is a further and important factor; the resulting delays are difficult to factor into the overall costs.

Some of the strands relating to the difficult commercial aspects of energy and waste were reinforced by Kris Wadrop (Solvert Ltd) in his talk on “Where’s there’s muck there’s n-butanol (and a range of other high value chemicals).” He described his experience in establishing a commercial process for the production of n-butanol and acetone by advanced anaerobic digestion. This process of biological fermentation allows organic waste to be used to produce valuable commodity chemicals and fuels. Thermochemical pretreatment, enzymatic hydrolysis, nutrient addition and sterilisation occur before first-stage digestion to produce n-butanol. 17000 tonnes of n-butanol are used in the UK every year; currently all of this is imported. Kris emphasised the financial difficulties encountered by a small company trying to break into a market dominated by large companies with long-term contracts for waste supply with organisations such as Tesco.

Raffaella Villa (Cranfield University) spoke on “Organic waste disposal: emissions and risks.” Her quote from Richard Buckminster Fuller is a wonderful expression of the lack of legal, political, financial and scientific imagination when it comes to the management of waste: “Pollution is nothing but the resources we are not harvesting. We allow them to disperse because we’ve been ignorant of their value.” Raffaella described her studies of the anaerobic treatment of municipal organic waste in composting facilities. Heat can be obtained from biogas production (methane and carbon dioxide), but odour problems are ubiquitous and like all odour issues, they are famous for their difficulties. Studies of bioaerosol emissions have shown that for the most part they declined quickly from the source, although anomalous and unexplained maxima in atmospheric bioaerosol concentrations were observed some distance away from the plant. As with all EfW processes, feedstock content and consistency are problematic; there are seventy different types of waste which can be used in an anaerobic digester, and seventy-five hazards associated with them. Risks can be reduced by excluding certain types of waste and by applying Quality Assurance/Quality Control codes of practice to feedstock sources, but this possibility seems remote currently. Raffaella finished with a brief reference to the use of algal biomass as a feedstock for biogas production.

Paul Williams entitled his Distinguished Guest Lecture “Fuels, chemicals and materials from waste.” He explained that a total of 288.5 million tonnes of waste per year are generated in the UK. This waste can be broken down into 86 million tonnes from mining and quarrying, 101 million tonnes from construction and demolition, 67.3 million tonnes from commerce and industry, and 31.5 million tonnes from households. The most important change in the disposal of this waste has been the move from landfall to recycling. Although there has been only a small growth in incineration, the development of new processes, changes in the availability of resources, and targets set by government policy have combined to bring renewed attention to the arguments in favour of EfW. Paul surveyed the basic thermal conversion technologies available ‒ incineration, gasification and pyrolysis ‒ and then dealt in more detail with pyrolysis.

The primary products of pyrolysis (oil, char and syngas) can be subdivided in terms of the secondary products and their use, namely fuel and chemical feedstock from syngas, refinery feedstock and chemicals from liquid fuel, and solid fuel, activated charcoal and soil improver from char. The amounts and range of these products can be manipulated by changes in the burn. For example, slow pyrolysis (hours to days at 400 °C) produces mainly charcoal, whereas fast pyrolysis (1 second burn) results in the production of liquid at 400 to 650 °C, liquid and syngas at 650 to 900 °C, and syngas at 1000 to 3000 °C. The reactor type (fluidised bed, entrained flow, rotary kiln, etc.) is also relevant. For example, the Ensyn process produces bio-oil using an entrained flow reactor for fast pyrolysis; bio-oil is a potential substitute for fuel oil or as feedstock for the production of synthetic gasoline and diesel, although the high acidity of bio-oil and its poor stability are two of several problems which exist. Commercial-scale pyrolysis operations include Metso Mineral Industries, Japan; Toshiba Mixed Plastics Pyrolysis, Japan; Splainex Ltd, Netherlands; Empyro, Netherlands; and Dynamotive, Canada.

The characteristic feature of gasification is the provision of limited amounts of oxygen. The syngas produced has a tar content associated with fine particles and SOx pollutants; line blockage and corrosion caused by the tar is problematic. This “dirty” gas is can be put straight into a boiler to produce steam. Such processes have a history of long acceptance in Japan. Although “dirty” syngas can be purified (e.g. by catalytic gasification), the commercial arguments for this are not robust.

Paul finally turned his attention to the “valorization” of waste. For example, 1000 million tyres are disposed of globally each year. The catalytic pyrolysis of tyres produces a highly aromatic oil (toluene, benzene, xylene). Plastics, composite plastics and carbon fibre materials can also be treated as specialised feedstock to produce products with specific and niche demand.

The range and detail of the presentations produced a picture of a vibrant area of development and opportunity; the urgent need for smarter methods of waste treatment and for the fiscal and legislative framework to be developed to support them was made abundantly clear. There seems to be no limit to the possibilities for fine-tuning the many and varied waste treatment processes available for local scale activity, but robust and proven technologies that can be built using conventional investment tools are few in the UK.